1. Field of the Invention
The present invention relates to methods of predicting ionospheric scintillation and in particular methods of providing a user of a Global Navigation Satellite System (GNSS) with information relating to the reliability of positioning data derived from individual satellites.
2. Description of the Related Art
The ionosphere is a region of the Earth's atmosphere, from about 80 to 1000 km above its surface, where significant numbers of free electrons and ions are present. The free electrons and ions are produced by solar ionizing radiation (extreme ultraviolet and X-ray radiations) and by collisions of energetic particles (solar wind) with the neutral particles at the upper atmosphere. Positive ions and electrons in the upper atmosphere are usually well mixed and form electrically neutral plasma.
The ionosphere can affect L-band, and therefore GNSS, signal propagation and can therefore be important for the accuracy of such systems. The effect of the ionosphere on GNSS signal propagation is a function of the Total Electron Content (TEC) along the signal path and the frequency of the signal. TEC is an integrated quantity that represents the number of free electrons in a column of 1 m2 cross-section along the signal path between satellite and receiver, expressed in TEC units with 1 TECU=1016 electrons/m2 corresponding to about 16 cm range error for the GPS L1 frequency (1575.42 MHz). Rapid fluctuation of TEC leads to phase scintillation of the signal. In case of severe phase scintillation, the carrier tracking loop in a GNSS receiver may have problems to track. For this reason, it is important to consider these effects.
Various manners of detecting ionospheric scintillation are presently used. These may rely on multi-frequency receivers monitoring e.g. the GPS L1 and L2 frequencies with differential phase measurement between the dual frequencies or may alternatively rely on dedicated scintillation receivers such as Septentrio PolaRxS™ and NovAtel FlexPak6™.
One service provided by the Australian Government Bureau of Meteorology is a GPS TEC derived scintillation map. This map is derived using data with a 1 Hz sampling rate from GPS receivers. The underlying quantity computed is the variance of the time derivative of the TEC over 1 minute intervals. This is commonly known as ROTI (Rate of TEC) and serves as a measure of present ionospheric scintillation since the disturbed ionospheric conditions that cause scintillation lead to short timescale variations in the TEC.
Another system is disclosed by US2009224969, which describes a method for determining the ionospheric error across a network of GNSS reference stations. The method relies on dual-frequency phase measurements in a geometry-free linear combination. The data is filtered for ambiguities and the characteristic parameters of the ionosphere. In combination with filter results from other combinations of phase measurements (ionosphere free combination), the physically-based model provides ambiguity resolution. The method uses limited prediction of scintillation in time and space by fitting of spherical harmonic functions.
Although measurement of the error due to ionospheric scintillation is presently possible, it would be desirable to provide alternative services allowing prediction of scintillation in time.